Toggle high contrast

Ventilation, ventilation, ventilation

Published date
How do we keep ourselves safe from Covid transmission indoors? Ventilation is one of the most important factors. 

We know that Covid is an airborne virus, meaning it is primarily spread through the air in tiny particles, known as aerosols. Aerosols are different to droplets, which are larger and can be spread from touching surfaces; they are breathed out by a person. That means anyone can spread them, unknowingly: you don’t need to be coughing or sneezing.

Aerosols are small, they can remain suspended in the air for hours. So if you’ve entered a room where someone who is infectious but not showing symptoms has been, even if they have already left, you might still breathe in a Covid-19 aerosol.  

 A combination of concentration, airflow, humidity and temperature, all contribute to whether the aerosol load will be infectious.  

This means ventilation, where we make sure the air is renewed and refreshed regularly, is an incredibly important method for reducing Covid transmission. 

Every workplace risk assessment should include aerosol transmission, and outline what steps are being taken to improve ventilation where necessary. 

Despite ventilation being one of the most effective ways to mitigate risk, union safety reps have told the TUC that it’s the one employers are least likely to be paying attention to. In some cases, we need to force ventilation onto the agenda through union education and action. 

CO2 monitors 
More employers are purchasing CO2 monitors, and some unions are making use of them to carry out safety inspections. These devices monitor how much CO2, which is breathed out by people, is in the air in a given space. The higher the level of CO2 is, the more poorly a space is ventilated. A CO2 monitor can’t tell you if you’re breathing in Covid, but it will tell you if you’re breathing in other peoples’ breath. The higher the reading, the more likely exposure will be. 

A key threshold to be aware of is 800 parts per million (ppm): if a CO2 monitor is consistently showing a room as reading above 800ppm, action must be taken to improve ventilation, or the area should be taken out of use.  

When using CO2 monitors, remember to:

  • Take the reading in the most poorly ventilated area of the room, for example do not take the reading next to an open window.  

  • Make a plan to consistently monitor, to improve accuracy and give a better picture for how the air quality is changing: taking readings throughout the day, or week, or adjusting frequency depending on how the space is being used.  

You can find more information on using CO2 monitors from the Health and Safety Executive. 

Improving ventilation

Your CO2 monitor doesn’t improve ventilation, it only gives you an accurate picture of whether you need to make improvements, and by how much. Further action that may need to  be taken can include: reducing the occupancy in a given indoor space, opening windows and doors, and using equipment which can be purchased, such as local air filtration units. 

Ventilation and air conditioning systems can help, but only those which do not recirculate air: any systems which recirculate air around a room or building must be switched off, as these risk spreading aerosols further, rather than filtering them with fresh air supply. Similarly, it’s worth noting that a desk fan which you might have on during hot weather is going to blow air from one part of a room to another: while it might keep you cool, it risks aiding aerosol spread. 


The easiest way to boost the fresh air supply in an indoor space is to keep windows or doors open. 

In some workplaces, opening a window will not be an option. With outside air comes other risks: cold temperatures, pollution, or contamination. In some cases, opening windows is simply not an option, for example in a maternity ward, a food factory, or where there are none. This is not a get-out for employers: they must be taking every effort to provide effective ventilation by other means. 

Air filtering units 

Where existing methods to improve ventilation are not to a safe standard, employers should be purchasing and providing air cleaning and filtering units. These are relatively cheap and highly effective in removing airborne virus from indoor room air.   

There are minimum specifications, including the requirement for HEPA (high efficiency particulate absorbing) filters, which remove up to 99% of aerosols. The number of filters required will depend on the size of a workspace. Six ACH (air changes per hour) is considered a safe level of ventilation, and can be achieved by a combination of methods. It is important to bear in mind, CO2 monitoring where air filters are being used will not give an accurate reading of air quality: this is because filtering units will remove virus aerosols, but not CO2 from the air.

Face coverings and face masks 

Wearing a face covering will reduce, but cannot eliminate, your risk of infection. 

This means mitigations such as mask-wearing (in particular, respirator masks at a FFP2 or FFP3 standard, which are effective for aerosol as well as droplet spread) are all the more important; along with effective ventilation. Keeping a good supply of fresh in indoor spaces can make a big difference in diffusing any potential Covid aerosols. Face masks are not a replacement for good ventilation: where possible, both are preferable.

Actions unions are taking include: 

  • Adopting a union position and demands to the employer, based on the situation in your workplace and the steps required to bring ventilation to a safe level.  

  • Safety reps carrying out inspections of indoor workspaces using CO2 monitors. 

  • Issuing a union improvement notice, or emergency advice including removing oneself from the hazardous environment if serious risk presents.  

  • Utilising union representation on any safety committees to raise the issue, and consultation in risk assessment process. 

  • Holding union member-wide meetings on the subject of ventilation, explaining the problem to members, and deciding on next steps including collective action to force changes to keep workers safe, and escalating matters through formal dispute procedures. 

See further resources:  

With thanks to Doctor Jonathan Fluxman for information for this blog. See  

Enable Two-Factor Authentication

To access the admin area, you will need to setup two-factor authentication (TFA).

Setup now